Introduction: Color Changing Night Joule Thief

Picture of Color Changing Night Joule Thief
I got this inquiry from a teacher who teaches high school kids electronics - can we have a cool kit like Aurora mini 18, in a simpler and more affordable format? I suggested Night Joule Thief, but the teacher felt that it wasn't sexy enough for the kids. (ur... you know what I mean.)

So I combined the multi-color LED and Joule Thief in one, and got Colour Night Joule Thief. It's a mood lamp version of Night Joule Thief.


Step 1: Features

Picture of Features

Here are the highlights of the Colour Night Joule Thief.

    * Compact & streamlined design
    * Uses only one AA battery (or any 1.5V battery you can hook up to)
    * Easily adaptable to different size batteries - hook up holes to attach home made clips
    * A multicolor/color changing LED
    * Automatic turn on via a light sensor (adjustable sensitivity level)
    * Energy efficient - works even with a run-down battery, down to 0.6V
    * Makes a great little mood light

Step 2: Technical Overview

Picture of Technical Overview
The circuit is minimally changed from Night Joule Thief. In fact there are only two parts added. You can refer to Night Joule Thief instructable for the theory of operation, etc.


The key difference from Night Joule Thief is that the white LEDs are replaced with a self color changing LED. This LED has a little chip inside that control three color LEDs also inside. It's incredible that a circuit like that can fit within a 5mm LED.
However the chip inside requires DC voltage to operate, but the original Joule Thief circuit produces pulsed DC voltage - the voltage swings high and low very quickly. So I added diode (D1) and a capacitor (C2) to rectify the output voltage of the boost circuit. Now the color changing LED gets about 3V of steady voltage to operate.

Parts List
1x CdS Photoresistor (rated 3k - 0.3M ohm) (CDS1)
1x 1k ohm (R1)
1x 100k ohm (R2)
1x 10k ohm (R3)
1x 50k ohm trim pot (VR1)
1x 22pF (C1)
1x 10uF (C2)
1x 470uH (L1) (anywhere between 22 - 470uH would work - might have to reduce the C1 value however)
1x 2N5401 or equivalent (Q1) (or just about any general purpose PNP transistor, such as PN2907, 2N3906, etc...)
2x MPSA06 or equivalent (Q2, Q3) (or just about any general purpose NPN transistor, such as PN2222A, 2N3904, 2N4401, etc...)
1x 1N4148 or equivalent (D1)
1x Color changing LED (D2) (I used "slow changing" type - use anything you want)
2x Battery Clips

Step 3: Assembly

Picture of Assembly
The assembly is very straight forward. Insert the parts into the PCB, and solder them. Start with smaller/lower profile components, and move on to larger/higher components. Follow the order listed below.

Parts List (in assembly order)
1x 1k ohm (Brown-Black-Red-Gold) (R1) - red line on the tape
1x 100k ohm (Brown-Black-Yellow-Gold) (R2) - green line on the tape
1x 10k ohm (Brown-Black-Orange-Gold) (R3)
1x 1N4148 or equivalent diode (D1)
1x Photoresistor (rated 3k - 0.3M ohm) (CDS1)
1x 50k ohm trim pot (VR1)
1x 22pF (C1)
1x 10uF (C2)
1x 470uH (L1)
1x 2N5401 or equivalent (Q1)
2x MPSA06 or equivalent (Q2, Q3)
1x Color changing LED (D2)
2x Battery Clips - attach from the bottom side

Transistors, diode, capacitor C2, and LED have polarities, so make sure to insert them in the correct orientation. One of the leads of diode D1 (left when you view from the top) sits close to the battery clip. Make sure to clip this lead short to avoid short circuit.
Battery holders need a bit of force to snap into the holes. They attach from the back side of PCB as you can see in the picture.

Once everything is soldered in place, double check the part placement, orientation and solder joints. Then insert a battery. The polarity is marked on the front side of PCB.
If you don't see the LED light up, don't worry. The room is probably too bright. Take a piece of black paper or tape and block the light from hitting CdS light sensor. (and/or darken the room) If the LED still don't come on, turn the trimmer (the little orange thing) with a screw driver, counter clockwise. This makes the sensor less sensitive to light, so the LED will come on by just placing the sensor under shade, or turning off the room light.

Step 4: PCB & Kit

Picture of PCB & Kit

If you are handy, you can etch your own PCB, and build this night light entirely DIY.

Otherwise, you can order the kit or the PCB.

You can purchase the kit from my website:

Also available at The Maker Shed:


ledartist (author)2012-11-28

This popular kit is now available at The Maker Shed!


colin55 made it! (author)2015-08-28

You don't need the PNP transistor. Here is the simpler circuit:

ledartist (author)colin552016-08-21

That's an interesting idea. However I think your circuit would drain battery very quickly when the ambient is bright? Since it keeps Q3 continuously on when the sensor is lit.

maximzodal (author)2015-02-06

This is a really unique application of a joule thief! The placement of the LEDs internally causes them to be slightly off center and the pattern on the ceiling of the different LEDs fading in and out or combining to change color is just fascinating. I wish I better understood how these LEDs work.

Using an inductor was so much easier than winding a toroid!

The kit from Maker Shed came together quickly, my only problem being a solder bridge between two of the three adjacent resistors. I always use a magnifying loop to check circuits at each step and caught it before going on. In hindsight I should have changed out the soldering tip for a smaller one.

Ironically, my only complaint is the LED is too bright! I don't understand electronics enough to figure how to make it dimmer. Is there a way to turn the brightness down?

Thanks for an interesting project!

raydar670 made it! (author)2014-07-12

I recently bought the kit from MakerShed and assembled it, but I found that a partially dead AA battery wasn't enough to power it (it's about 1.34V). I checked the voltage drop across the LED and it was only about 0.85V which wasn't enough to power the LED. I then decided to use my Arduino as a power supply and hook up 3.3V to the Joule Thief. It does light up, but the voltage drop across the LED was about 2.5V. So instead of boosting the voltage the circuit is lowering it. If I were to guess I think something is wrong with the capacitor or inductor. Is it possible I place the inductor in backwards reversing the polarity and changing the value? Also according to the parts list it's a 100uH inductor while this instructible states it needs a 470uH inductor. Not sure if that helps, but I'd love to get this working. Thanks!

raydar670 (author)raydar6702014-08-25

I got it working! I made a simple mistake of soldering one of the resistors in the wrong place. Once that was fixed it works fine with just an AA battery. The reason it still worked with straight 3.3V was because the way I had the circuit set up all the intermediate circuitry between the LED and the battery were bypassed so I was powering the LED directly with no boosting involved.

ledartist (author)raydar6702014-09-27

Glad to hear that you got it working. Sorry I could not respond sooner.
Regarding the inductor value - there's no performance change between 100 and 470 uH. Although in theory 100 uH should perform a tad bit better, due to it's lower DC resistance.
Also, inductor polarity doesn't matter with this circuit (in most circuit it doesn't. It usually only matter when you have multiple inductors nearby.)

raydar670 (author)ledartist2014-09-27

Thanks for the reply! And just one more update (because I can't seem to edit my previous post). I had switched one of the transistors with another one, so there was no switching going on at all (it wasn't a resistor that was the issue). That's why it would work at 3.3V and not the ~1V. Also, thanks for the clarification on the inductors.

ledartist (author)raydar6702014-09-27

I suspect that transistor got damaged, possibly by having a wrong resistor and applying higher voltage. It's never a good idea to apply higher voltage than the circuit is designed for, _especially_ when something is not working.

JonathanC2 (author)2014-08-06

so uh.. how long will this battery power the led?

pocokk (author)2014-07-08

Thanks for the pulsed DC voltage hints. I was stuck at only Red Color using slow flashing RGB LED. Now I know why and problem solved. I hereby salute you smart guy ! haha. Thanks Pal.

Robert Powell (author)2014-06-25

You are so helpful! I don't want to buy it so i did this, I already had a joule thief on my breadboard but my 3 pin color changing led wouldn't work cuz the fast switching so I put a shotky diode and a 100 uf cap and it works so good! Thanks!

Robert Powell (author)2014-06-25

You are so helpful! I don't want to buy it so i did this, I already had a joule thief on my breadboard but my 3 pin color changing led wouldn't work cuz the fast switching so I put a shotky diode and a 100 uf cap and it works so good! Thanks!

TxFireRanger (author)2014-01-10

Anyone know the value of the LDR? And which ones will work?

douglas marcelo (author)2013-12-29

coloquei esse circuito na minha pipa hehehe vlw pela ajuda ñ saibia qual usava

jayz01982 (author)2013-04-02

do you have branch here in the philippines? so that i can order it here

ledartist (author)jayz019822013-04-03


I don't have a branch or reseller in Philippines, but I ship there. Please go to, and shop.


lgore93 (author)2012-03-02

Do you think I could maybe replace the PCB with a couple of popsicle sticks glued together? I know that sounds kind of ghetto, but I'm looking for a very inexpensive project to do in class. It would be much cheaper if I could make a working model on popsicle sticks. I know it wouldn't look as professional, but things would be much easier.

ledartist (author)lgore932012-03-02

Yeah, you should definitely try that. Please post the result!


lgore93 (author)ledartist2012-03-02

Okay thanks! One more question which is probably obvious... The smallest photoresistor I can buy from the site I usually order from is 5k-0.5M ohm. Would the circuit work with this difference? I realize 2k ohms is a significant amount, but is it really that much on this scale? I admit that I know much less of electronics than I would like to, but I love learning and building new things. Thanks for your help.

ledartist (author)lgore932012-03-03

That will probably work. I'd just try it, maybe on a breadboard.


lgore93 (author)ledartist2012-03-24

My parts came in yesterday, and I built it. It's not the prettiest thing in the world, but it works very well! I decided to use a piece of cardboard instead of popsicle sticks since it allowed for some less than... perfect wiring. Also, I found out I was out of electrical tape so I had to substitute duct tape in. Haha

hussainsk (author)lgore932012-06-23

i was prepared to electromagnet 9v battery how much turns required 24 gauge wire

waterlubber (author)lgore932012-06-05

Mechiver electronics! Woot!

ledartist (author)lgore932012-03-26

Umm... You are good.

Nano_Burger (author)2012-06-12

LED Artist,

Can you use different voltage batteries with this circuit? O have 3v, 6v and even 12v photo batteries I'd like to drain off as well. They are probably not up to those voltages since they became too weak for their particular device so a few volts short.

ledartist (author)Nano_Burger2012-06-12

If the voltage is below 1.6V or so, then it's safe to connect to this circuit.

ciliar (author)2012-03-25

Aki, you are a star - soon to put Colour Joule Theif into action! Keep you posted!

ledartist (author)ciliar2012-03-25

Yes keep me posted.


markallie (author)2012-03-13

In the pdf version of the schematic an MPSA05 is listed as a choice for the PNP transistor, Q1. It is an NPN transistor.

ledartist (author)markallie2012-03-13

Thank you for pointing this out. I've corrected the error.


GASSYPOOTS (author)2012-02-23

how bout a self charging/ no batterys needed after 1st charge

64bitgenius (author)GASSYPOOTS2012-02-27

unfortunatly, I don't think thats realistically possible. the reason for that is that it is going to lose energy no matter what. if it were 100% efficient, it would be possible, but it's not.

sageserver (author)2012-02-23

Can another LED be used in place of D1? or is the voltage difference too much? Very instructive instructable.

ledartist (author)sageserver2012-02-23

D1 is there to rectify the voltage, so it has to be regular diode. LEDs have too high of forward voltage drop.
If you wan to drive two LEDs, you can simply add another LED in series to D2. The brightness will go down a bit, but it will still work.


About This Instructable




Bio: I am an electronic artist living in Brooklyn, NY. I work with LEDs and microcontrollers to create beautiful objects.
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